Fluid responsive vent control valves are well known in the art for venting fuel vapor from a vehicle fuel tank to the atmosphere or, more recently, to a vapor recovery system such as a carbon canister. Such valves typically vent the interior of the fuel tank to the carbon canister when the fuel in the tank is below the level of the valve, and are closed by a float when the fuel level rises or in a rollover situation. Closing the valve in response to rising fuel level prevents the overflow of liquid fuel into the carbon canister, for example during refueling. Such valves can be employed as onboard vapor recovery control valves, or as rollover vent valves. If used for onboard vapor control, closure is in response to a full fuel level in addition to surge or rollover and results in a pressure head within the fuel tank and filler pipe to operate automatic shutoff apparatus built into the fuel fill nozzle.
In U.S. Pat. No. 4,753,262 to R. Bergsma, issued Jun. 28, 1988, a rollover vent valve having a venting outlet closed by a peel-away valve disc is disclosed. When liquid fuel reaches the valve a float member is raised to force the valve disc against the vent outlet, thereby closing it to prevent fuel from reaching the canister. In order to ensure positive opening of the valve when the fuel level drops, the float and the structure connecting it to the valve plate operate to circumferentially lever or peel the valve plate off the vent outlet.
As noted in the above-cited patent, increasing the rate of vapor flow from the fuel tank through the valve to the canister requires an increase in the area of the valve vent outlet. Providing the valve with a large vent outlet increases the force acting to maintain the valve in the closed position once the fuel level drops; i.e., the greater the area of the vent outlet, the greater the closing force on the valve plate caused by the tank/canister pressure differential. With a large vent outlet and corresponding valve plate subjected to a substantial tank/canister pressure differential, the float can "hang-up" despite a drop in the fuel level. The valve would then remain closed, resulting in a rapid and undesirable buildup of pressure in the unvented tank.
An alternate approach for opening a valve member from a relatively large vent outlet against the pressure differential thereacross is to provide a flexible, ribbon type valve member to achieve a continuous peeling effect when the float pulls it away from the vent outlet. U.S. Pat. No. 4,770,201 to Zakai illustrates a float-operated faucet or air-purge valve having a flexible membrane valve member progressively detached or peeled from a vent outlet by a float as liquid level drops.
While the Zakai patent generally illustrates a flexible membrane valve with a float-operated peeling action, it is not particularly adapted to the requirements and problems found in automotive fuel tank venting applications. For example, space limitations in vehicle fuel tanks favor compact, small diameter valves; this makes coaxial float/vent outlet arrangements preferable to offset structure as shown in Zakai. The pressure differential and flow conditions in a vehicle fuel tank, and the relationship of these pressure conditions to liquid level, can create extra difficulties with respect to initially opening or peeling the valve member from the vent outlet, and further in wrinkling or bunching up at various stages of operation. Also, it is increasingly desirable to provide an initial shutoff indication to the person refueling the tank, with allowance for a few "round off" clicks on the filler nozzle for final shutoff. These and other problems encountered with peel-away valves in the fuel tank environment are solved by the following invention.